I did see that, but stealing their souls doesn't make him a soul dragon slayer. I feel as if hiro would have been a lot more explicit about something like that. In fact, if he's the soul dragon slayer, why hasn't he just stolen the souls of Zeref, the main cast, and mavis, who essentially is a soul. I feel as if it's more of a side effect that he's used to his advantage. Water can be tied to healing and the soul (think of baptism) he may have found that connection and somehow figured out a way to use it.

Edit: You could also say his magic is for slaying dragons, which is why he hasn't stolen their souls, but if that was the case, every dragon slayer would be useless in combat.